World No.12 Thailand are last with two points — from their 3-2 upset win against Serbia in the opening week — in the 12-team elite division after six matches in the previous two preliminary tournaments in South Korea and Hong Kong.
The Thais will begin their campaign at Hua Mark Indoor Stadium against the Dominican Republic on Friday and then face No.2 United States on Saturday and No.1 Brazil on Sunday.
"We are not fully fit,'' said Somporn Chaybangyang, president of the Volleyball Association of Thailand.
"Some players including Pleumjit Thinkaow are injured while others are exhausted after tough matches in the first two weeks.''
Thailand's best chance for a win is against No.8 Dominicans, he said.
"We are underdogs against the Dominicans but we hope support from our fans will give us a boost,'' he said.
The stadium has a capacity of 6,500 and tickets for the three matches have been sold out.
Brazil are leaders in the preliminary round with 18 points from six wins, while the US are sixth with nine points from a 3-3 record and the Dominicans 11th with three points -- one place and one point ahead of the Thais.
While Thailand have been unconvincing in this year's World Grand Prix, Somporn said that the association was satisfied with the performance of young players who may shine at next month's Asian Games in Incheon, South Korea.
It is believed that Thailand would be relegated from the top division next year if they fail to finish in the top-10 in the current campaign but Somporn said the rules were unclear.
However, he admitted that it would be a big loss for the team and Thai fans if the national side fails to remain in the elite group.